1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing piezoelectric devices. More particularly, the present invention is directed to an automated, high-volume method and apparatus for manufacturing multi-layered high deformation piezoelectric actuators and sensors.
2. Description of the Prior Art
The present invention is a unique method and apparatus for automatically manufacturing piezoelectric actuators and sensors, principally pre-stressed high deformation actuators and sensors. The disclosed invention provides a method of manufacturing high deformation actuators which is fast, reliable, precise and easy as compared with prior manufacturing methods.
Piezoelectric materials change shape when a voltage potential is applied across their faces. Piezoelectric materials used in conventional configurations have limited application because of the relatively small amount of displacement which the piezoelectric material undergoes during electrical excitation. In order to increase the amount of displacement which can be developed by the piezoelectric material (typically a thin ceramic wafer) the material may be "pre-stressed", such that the ceramic wafer is normally under compression when unenergized. Prior methods of pre-stressing the ceramic wafer include bonding a metallic foil (for example aluminum, stainless steel or the like), under tension, to a major face of the ceramic wafer with an adhesive, thus creating what is known as "THUNDER" (THin layer composite UNimorph ferroelectric Driver and sEnsoR) as disclosed in U.S. Pat. No. 5,632,841. Other methods, such as the "Rainbow" method as disclosed in U.S. Pat. No. 5,471,721, use a chemical reduction process to pre-stress the ceramic wafer. The present invention provides a method and apparatus for producing pre-stressed piezoelectric actuators such as THUNDER and the like. The following disclosure principally describes the preferred embodiment of the invention and its use in manufacturing THUNDER. It will be understood, however, that the present invention, or modifications thereof, may be used to manufacture other types of multi-layer piezoelectric actuators and sensors.
Prior methods of manufacturing THUNDER include inefficient, low output methods which rely heavily on human labor. The most common prior method of manufacturing THUNDER actuators is as follows: THUNDER actuators are constructed in a "sandwich" configuration with each actuator having a plurality of layers, including first and second metal pre-stress layers, first and second adhesive layers, and a PZT ceramic wafer having electrodes disposed on both of its major faces. Initially, all of the layers are manually cut to their desired shape. A razor blade or similar instrument is used to cut the ceramic wafer; and a paper cutter, scissors or a razor blade is typically used to cut the adhesive and metal pre-stress layers to size.
Before the "sandwich" can be assembled, the two major faces of the ceramic wafer, one major face of the first metal pre-stress layer and one major face of the second metal pre-stress layer are sprayed with a primer coating of a thermoplastic adhesive, such as "LaRC-SI", using an air brush or the like. LaRC-SI is a soluble imide developed by the National Aeronautics and Space Administration that is manufactured by NASA in accordance with the process disclosed in U.S. Pat. No. 5,639,850. Initially, one side of the ceramic wafer is sprayed with LaRC-SI. The coated ceramic wafers are placed on a release cloth-covered aluminum tray. The aluminum tray, release cloth and ceramic wafers are placed in an oven at approximately 70 deg. C., where they remain until the LaRC-SI dries. The tray and its contents are subsequently removed from the oven, and the LaRC-SI coating process is repeated a second time for the same side of the ceramic wafer. After the second coat is dry, the ceramic wafers are turned over and two coats of LaRC-SI adhesive are applied to the opposing major face using the above described process. The same process is then repeated for the first and second metal pre-stress layers, however only one major face of the metal pre-stress layers is coated. Because LaRC-SI is a dielectric, and in a finished THUNDER actuator the adhesive layer is disposed between a metal pre-stress layer and the ceramic wafer, it is sometimes necessary to roughen a major face of the metal prestress layers using sandpaper so that intermittent electrical contact is made between the metal prestress layers and the electrodes.
After the LaRC-SI coating on the ceramic wafers and the first and second metal pre-stress layers are dry, the "sandwich" is ready for assembly. The first metal pre-stress layer, which is usually the bottom layer in the "sandwich", typically comprises steel, stainless steel, beryllium alloy or other metal. Placed adjacent the first pre-stress layer in the "sandwich" is the first adhesive layer which is typically LaRC-SI material in a thin film form. The PZT piezoelectric ceramic wafer which is electroplated on its two opposing faces is placed on top of the first adhesive layer. A second adhesive layer, also comprising LaRC-SI material or the like, is positioned on top of the ceramic wafer, and a second metal pre-stress layer, which typically comprises aluminum foil or the like, is placed on top of the second adhesive layer thereby completing the "sandwich". As the layers are stacked in the desirable configuration a "dot" of glue is placed between each adjacent layer to prevent slippage of adjacent layers of the "sandwich" during the manufacturing process. Prior THUNDER actuators have been constructed using various numbers of adhesive layers and/or metal pre-stress layers, depending on the desired pre-stressing characteristics.
The "sandwich" building process is repeated until a desirable number of composite structures have been assembled. Each assembled composite structure is placed on a heating tray. The heating tray comprises an aluminum plate, a first layer of fiberglass, and a first layer of release cloth. The first layer of fiberglass is positioned on top of the aluminum plate, and the first layer of release cloth is placed on top of the first layer of fiberglass. The composite structures are positioned on the heating tray, and a second layer of release cloth is placed on the composite structures. A second layer of fiberglass is placed on the second layer of release cloth. A heat resistant sealant tape is disposed around the perimeter of the heating tray to hold the first and second layers of release cloth, the first and second layers of fiberglass and the composite structures in place. A sheet of KAPTON.TM. as manufactured by the DuPont Company, is placed over the secured heating tray, and the entire assembly is placed in an autoclave. A vacuum line is inserted under the KAPTON sheet; and the KAPTON sheet pulls the composite structures against the heating tray as a vacuum is drawn through the vacuum line.
While in the autoclave, the ceramic wafer, the first and second adhesive layers and the first and second pre-stress layers are simultaneously heated to a temperature above the melting point of the adhesive material (typically several hundred degrees Fahrenheit). Due to the relatively large mass of the autoclave, it may take several hours to heat the entire inner chamber to a sufficient temperature. The temperature is then maintained above the LaRC-SI melting point for approximately an hour. Natural convection currents, set up within the chamber, transfer heat to the individual composite structures. In some situations, if natural convection is not sufficient, forced convection, using fans or pumps are used. After sufficient heating, the autoclave and the composite structures are allowed to cool, thereby re-solidifying and setting the adhesive layers. The cooling process typically takes several hours due to the high temperature within the autoclave. During the cooling process the ceramic wafer becomes compressively stressed, due to the higher coefficient of thermal contraction of the materials of the pre-stress layers than for the material of the ceramic wafer. Also, due to the greater thermal contraction of the laminate materials (e.g. the first pre-stress layer and the first adhesive layer) on one side of the ceramic wafer relative to the thermal contraction of the laminate materials (e.g. the second adhesive layer and the second pre-stress layer) on the other side of the ceramic wafer, the ceramic wafer deforms in an arcuate shape having a normally concave face and a normally convex face.
A strong bond between adjacent layers of a finished THUNDER actuator is critical. This bond is accomplished in the prior art by drawing the KAPTON sheet down onto the composite structures during the heating process using the vacuum line. Nitrogen is injected into the autoclave during the heating process to pressurize the autoclave. The pressure placed on the composite structures by the sheet of KAPTON and the pressurized nitrogen atmosphere, while the LaRC-SI is in a liquid state, aids in pressing the composite structures together and producing a substantial bond between adjacent layers.
Subsequent to cooling the autoclave sufficiently, the heating tray and its contents are removed from the autoclave, and each THUNDER element is then removed from the tray by hand. Each THUNDER element is then electrically polarized at a "poling station" by applying a relatively high voltage potential between the opposing major faces of the THUNDER element. The polarization process in the prior art typically takes approximately two minutes for each individual THUNDER element.
The above described prior THUNDER manufacturing process is time consuming, inefficient, and of relatively low quality as compared to what is achieved in the present invention. In prior THUNDER manufacturing processes tight tolerances cannot be maintained, and quality control is at a minimum. Manual cutting of each layer of the THUNDER device takes a great deal of time and is not as accurate as can be accomplished by machine.
Another problem inherent with this prior method of THUNDER production relates to the use of an autoclave to pressurize and heat the ceramic, adhesive, and metallic materials. An autoclave is not energy efficient because large quantities of heat are required to raise the temperature of the mass of the autoclave to desirable levels. The heating process is time consuming because in order to raise the temperature of a single "sandwich" the entire autoclave chamber must be heated, and the heat must be transferred from the autoclave chamber atmosphere to the KAPTON by convection, and then the heat must be transferred by conduction through the KAPTON (an insulator) to the "sandwich". A comparable amount of time is required to allow the autoclave to cool to a safe temperature before removing the THUNDER devices from the autoclave.
In addition, because of the tendency of the cooling "sandwich" to deform into an arcuate shape (due to the differences in coefficients of thermal expansion of the layers of the "sandwich"), it is necessary to release pressure on the "sandwich" in a controlled manner during the cooling process. It is desirable that the releasing of pressure on the "sandwich" be controlled in accordance with temperature of the "sandwich" as it cools down. However, because of the use of an autoclave to convectively heat (and subsequently cool down) the "sandwich" it is very difficult to determine the exact temperature of the "sandwich" at any given time, unless the autoclave atmosphere is heated up (and cooled down) very slowly.
Furthermore, due to temperature variations from one location to another within the autoclave, the temperature of one "sandwich" may be different from that of another inside of the autoclave. However, because the pressure on all of the "sandwiches" in the autoclave is the same (i.e. corresponding to the chamber atmosphere's pressure), it is not possible to release the pressure on individual THUNDER "sandwiches" at different times in accordance with the respective temperatures of each "sandwich".
Accordingly, it would be desirable to provide a method and apparatus for high volume, high precision, high speed manufacturing of piezoelectric actuators which limits human involvement and maximizes efficiency.